This invention relates to a biaxially oriented polyester film. More specifically, it relates to a biaxially oriented polyester film which has excellent windability, abrasion resistance and running durability, can be produced at a low cost and is useful as a base film for a high-speed duplicator magnetic recording medium.
A biaxially oriented polyester film typified by a polyethylene terephthalate film is widely used as a base film for a magnetic recording medium such as a magnetic tape owing to its excellent physical and chemical properties.
Of applications to magnetic tapes, the production of tapes on which software such as movies has been recorded in advance shows remarkable growth lately. Heretofore, such software has been duplicated from a master VTR to several thousands of VTRs at a high speed. However, in recent years, a unrecorded magnetic tape is superposed on a mirror master tape on which software has been recorded and a magnetic field and temperature are applied to the superposed tapes to effect a transfer of the recorded software. In this case, the duplicating speed is extremely high, and in consequence, properties required for the magnetic tape have been changed and are changing, and new properties required for the high-speed duplicator base film are increasing. For example, there are proposed windability at the time when the tape is wound up at a high speed; a surface smoothness for obtaining close contact with the master tape and improving electromagnetic conversion characteristics; improvement of abrasion resistance against a guide roll with which the film comes into contact at the time of duplication; and the like.
Conventionally, with a view to improving windability, inert particles having a large particle diameter are contained in the base film of a magnetic recording medium to improve its air squeeze properties. However, when a large amount of inert particles having a large particle diameter are contained, the surface of a magnetic layer is pushed up by protrusions or protrusions formed by large-sized inert particles are transferred to the surface of a magnetic layer when films are wound up together, thereby greatly deteriorating the surface properties of the magnetic layer of a magnetic tape. Further, the large-sized fine inert particles are shaved off by their contact with a guide roll to generate white dusts, thereby causing a drop-out.
In addition, environment for the abrasion resistance and running durability of a magnetic tape is becoming worse and worse by the use of a surface finished cassette half or guide pin or by the use of a plastic guide pin for a cost reduction. Under the circumstances, improvements of the above properties have been desired much more than before.
JP-A 3-239731 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses a biaxially oriented polyester film containing crosslinked polymer particles (A) whose weight reduction rate after 30 minutes of a treatment at 300.degree. C. is not more than 30% by weight and which has an average particle diameter of 0.05 to 3 .mu.m and contains ethylene glycol units, particles (B) whose average particle diameter is less than 0.7 time that of the particles (A) and/or particles (C) whose average particle diameter is not less than 1.4 time that of the particles (A). This publication fails to disclose a biaxially oriented polyester film containing all of the particles (A), (B) and (C) simultaneously.
JP-A 7-169031 discloses a biaxially oriented laminate polyester film. One layer (layer A) constituting this laminate polyester film may contain 0.1 to 0.5% by weight of small-sized particles, which are selected from amorphous titanium oxide and type titanium oxide, having an average particle diameter of 0.05 to 0.3 .mu.m in addition to 0.05 to 0.5% by weight of medium-sized inert particles having an average particle diameter of 0.4 to 0.9 .mu.m and 0.001 to 0.05% by weight of large-sized inert particles having an average particle diameter of 1.0 to 1.5 .mu.m. The layer A is characteristically as thin as 0.5 to 2.0 .mu.m. This publication fails to disclose a single-layer film.
JP-A 7-252408 discloses a biaxially oriented polyester film composed of a polyester composition comprising 0.05 to 3% by weight of inert particles A having a Moh's hardness of 6 or more and an average particle diameter of 0.05 to 0.5 .mu.m, 0.05 to 3% by weight of inorganic particles B having a Moh's hardness of below 6 and an average particle diameter of 0.1 to 1.0 .mu.m which is larger than that of the particles A, and 0.05 to 3% by weight of crosslinked polymer particles C having an average particle diameter of 0.1 to 1.0 .mu.m which is different from that of the particles B by not more than 0.3 .mu.m. This biaxially oriented polyester film contains large-sized crosslinked polymer particles in a relatively large amount (a lower limit of 0.05% by weight).
It is an object of the present invention to provide a biaxially oriented polyester film.
It is another object of the present invention to provide a biaxially oriented polyester film which is excellent in windability, smoothness of base surface, abrasion resistance and the like and has such abrasion resistance and running durability to be usable for even an inferior cassette half, guide pin or the like.
It is still another object of the present invention to provide a biaxially oriented polyester film which is useful as a base film for a high-speed duplicator magnetic tape.
It is a further object of the present invention to provide a biaxially oriented laminate polyester film which has the same characteristic properties as those of the above biaxially oriented polyester film of the present invention and also excellent electromagnetic conversion characteristics and total performance and can be produced at a low cost.
Other objects and advantages of the present invention will become apparent from the following description.
According to the present invention, the above objects and advantages of the present invention can be attained by a biaxially oriented polyester film (may be referred to as "single-layer polyester film of the present invention" hereinafter) which consists of a polyester composition comprising:
a) 0.0005 to 0.03% by weight of crosslinked polymer particles having an average particle diameter d.sub.A of 0.8 to 1.6 .mu.m, PA1 b) 0.1 to 0.8% by weight of inert particles having an average particle diameter d.sub.B of 0.4 to 0.8 .mu.m, and PA1 c) 0.05 to 1.0% by weight of inert inorganic particles having an average particle diameter d.sub.C of 0.01 to 0.3 .mu.m and a Moh's hardness of not less than 7, and which satisfies the following equations (1) and (2): EQU 1.5.ltoreq.d.sub.A /d.sub.B .ltoreq.3 (1) PA1 wherein d.sub.A and d.sub.B are defined as above, EQU SRz/SRa.gtoreq.20 (2) PA1 wherein SRz is a three dimensional 10-points average roughness and SRa is a three dimensional center plane average roughness of a film surface.
Secondly, the above objects and advantages of the present invention is attained by a biaxially oriented laminate polyester film (may be referred to as "laminate polyester film of the present invention" hereinafter) which comprises a first polyester layer and a second polyester layer which is the above single-layer polyester film of the present invention formed on at least one side of the first polyester layer.